U.S. patent number 7,048,823 [Application Number 10/190,389] was granted by the patent office on 2006-05-23 for acrylic films prepared by coating methods.
This patent grant is currently assigned to Eastman Kodak Company. Invention is credited to Marcus S Bermel.
United States Patent |
7,048,823 |
Bermel |
May 23, 2006 |
**Please see images for:
( Certificate of Correction ) ** |
Acrylic films prepared by coating methods
Abstract
A method of film fabrication is taught that uses a coating and
drying apparatus to fabricate resin films suitable for optical
applications. In particular, acrylic films are prepared by
simultaneous application of multiple liquid layers to a moving
carrier substrate. After solvent removal, the acrylic films are
peeled from the sacrificial carrier substrate. Acrylic films
prepared by the current invention exhibit good clarity and low
birefringence.
Inventors: |
Bermel; Marcus S (Pittsford,
NY) |
Assignee: |
Eastman Kodak Company
(Rochester, NY)
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Family
ID: |
29423132 |
Appl.
No.: |
10/190,389 |
Filed: |
July 3, 2002 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20030215621 A1 |
Nov 20, 2003 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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60381931 |
May 20, 2002 |
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Current U.S.
Class: |
156/247;
427/372.2; 427/177; 427/162 |
Current CPC
Class: |
B29C
48/08 (20190201); B29C 41/32 (20130101); G02B
5/3083 (20130101); C08J 7/043 (20200101); B29C
48/0014 (20190201); B29C 48/21 (20190201); C08J
7/0427 (20200101); C08J 5/18 (20130101); B29C
41/26 (20130101); B32B 27/30 (20130101); B29D
7/01 (20130101); B29K 2033/12 (20130101); B29L
2009/00 (20130101); B29K 2027/00 (20130101); B32B
2307/734 (20130101); B29K 2001/00 (20130101); B29K
2001/12 (20130101); B29K 2033/08 (20130101); B29C
48/304 (20190201); B32B 2307/40 (20130101); B29K
2069/00 (20130101); C08J 2367/02 (20130101); Y10T
428/24942 (20150115); B29K 2705/02 (20130101); C08J
2333/08 (20130101); B29K 2029/00 (20130101); C08J
2433/00 (20130101); B29C 48/35 (20190201); B29K
2995/0026 (20130101); B29C 48/154 (20190201); B29K
2067/00 (20130101); B29K 2081/06 (20130101); B29L
2011/00 (20130101) |
Current International
Class: |
B29C
63/00 (20060101); B05D 3/02 (20060101); B05D
5/06 (20060101); B29D 7/01 (20060101) |
Field of
Search: |
;156/184,242,246-248,278,282,289,307.1,307.7,323,324,344
;427/162,164,177,333,372,372.2,374.2,420 ;428/46 |
References Cited
[Referenced By]
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0 154 108 |
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EP |
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0 481 273 |
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EP |
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JP |
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Other References
Solvent film casting--a versatile technology for specialty films,
Ulrich Siemann, Luigi Borla, LOFO High Tech Film GmbH, D-79576 Weil
am Rhein, Germany, pp. 1-4, Feb. 19, 2001, DRS/bo. cited by other
.
Surfactants: Static and Dynamic Surface Tension by Y.M. Tricot in
Liquid Film Coating, pp. 99-136, SE Kistler and PM Schweitzer,
Editors, Chapman and Hall (1997). cited by other .
Handbook of Plastics, Elastomers and Composites, CA Harper Editor,
McGraw-Hill, Inc. (2000), pp. 6.66-8. cited by other .
JP Abstract 03-252625. cited by other .
JP Abstract 09-216241. cited by other .
JP Abstract 62-229205. cited by other .
Japanese Patent Abstract 5-064821. cited by other .
Japanese Patent Abstract 63-13100. cited by other .
Japanese Patent Abstract 9-52240. cited by other .
Japanese Patent Abstract 61-005986. cited by other .
Japanese Patent Abstract 7-186163. cited by other .
Japanese Patent Abstract 05-059310. cited by other .
Japanese Patent Abstract 57-059961. cited by other .
Japanese Patent Abstract 59-047268. cited by other .
Japanese Patent Abstract 11-254594. cited by other .
Japanese Patent Abstract 2002-090541. cited by other .
Japanese Patent Abstract 62-064514. cited by other .
Japanese Patent Abstract 10-080231. cited by other .
Japanese Patent Abstract 11-005851. cited by other .
Japanese Patent Abstract 2000-047012. cited by other .
Surfactants: Static and Dynami Surface Tension by Y.M. Tricot,
"Liquid Film Coating, Scientific Principles and Their Technological
Implications", pp. 99-136, Edited by: S.F. Kisler & P.M.
Schweizer, Chapman & Hall, 1997. cited by other.
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Primary Examiner: Fiorilla; Chris
Assistant Examiner: Chan; Sing P.
Attorney, Agent or Firm: Bocchetti; Mark G. Leipold; Paul
A.
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This is a 111A Application of Provisional Application, Ser. No.
60/381,931, filed on May 20, 2002
Claims
What is claimed is:
1. A coating method for forming an acrylic film comprising the
steps of: (a) applying a liquid acrylic/solvent mixture onto a
moving, discontinuous carrier substrate; and (b) drying the liquid
acrylic/solvent mixture to less than 5% solvent so as to
substantially remove the solvent yielding a composite of an acrylic
film adhered to the discontinuous carrier substrate, winding the
composite into at least one roll before the acrylic sheet is peeled
from the discontinuous carrier substrate, the acrylic film being
releasably adhered to the discontinuous carrier substrate thereby
allowing the acrylic film to be peeled from the discontinuous
carrier substrate, wherein the acrylic film has a thickness in the
range of from about 1 to about 500 .mu.m, the acrylic film has an
in-plane retardation that is less than 5 nm, and the acrylic film
is adhered to the carrier substrate with an adhesive strength of
less than about 250 N/m further comprising the step of: delivering
the composite to a user of the acrylic film, the carrier substrate
acting as a protective support for the acrylic film prior to the
acrylic film being separated from the substrate carrier, cuffing
the composite and then removing the acrylic film.
2. A coating method as recited in claim 1 wherein: the liquid
acrylic/solvent mixture is applied using slide bead coating die
with a multi-layer composite being formed on a slide surface
thereof.
3. A coating method as recited in claim 2 wherein: the viscosity of
each liquid layer of the multi-layer composite is less than 5000
cp.
4. A coating method as recited in claim 2 wherein: an uppermost
layer of the multi-layer composite contains a surfactant.
5. A coating method as recited in claim 2 wherein: an uppermost
layer of the multi-layer composite contains a fluorinated
surfactant.
6. A coating method as recited in claim 2 wherein: an uppermost
layer of the multi-layer composite contains a polysiloxane
surfactant.
7. A coating method as recited in claim 1 wherein: the carrier
substrate is polyethylene terephthalate.
8. A coating method as recited in claim 1 wherein: the carrier
substrate has a subbing layer applied to the coated surface.
9. A coating method as recited in claim 1 wherein: the first drying
section is operated at a temperature of less than 95.degree. C.
10. A coating method as recited in claim 9 wherein: the drying step
is initially performed at a temperature in the range of from about
25.degree. C. to less than 95.degree. C.
11. A coating method as recited in claim 9 wherein: the drying step
is initially performed at a temperature in the range of from about
30.degree. C. to about 60.degree. C.
12. A coating method as recited in claim 7 wherein: the drying step
is initially performed at a temperature in the range of from about
30.degree. C. to about 50.degree. C.
13. A coating method as recited in claim 1 further comprising the
step of: including a plasticizer in the liquid acrylic/solvent
mixture.
14. A coating method as recited in claim 1 wherein: the acrylic
film has an in-plane retardation of less than 1.0 nm.
15. A coating method as recited in claim 14 wherein: the optical
resin film has a light transmittance of at least about 85 percent
and a haze value of less than about 1.0 percent.
16. A coating method as recited in claim 15 wherein: the optical
resin film has an average surface roughness of less than about 50
nm.
17. A coating method as recited in claim 1 further comprising the
step of: applying at least one additional acrylic layer to the
composite after the drying step.
18. A coating method as recited in claim 1 further comprising the
step of: using the acrylic film to form a light polarizer.
19. A coating method as recited in claim 1 wherein: the optical
resin film has an average surface roughness of less than about 100
nm.
20. A coating method as recited in claim 1 wherein: the optical
resin film has an average surface roughness of not more than about
1 nm.
Description
FIELD OF THE INVENTION
This invention relates generally to methods for manufacturing resin
films and, more particularly, to an improved method for the
manufacture of optical films, and most particularly, to the
manufacture of acrylic films used in optical devices such as light
filters, liquid crystal displays and other electronic displays.
BACKGROUND OF THE INVENTION
Acrylic polymers are used to produce films that are noted for
outstanding optical clarity. Acrylics also have good resistance to
ultra violet light and good weather resistance. These attributes
make acrylic-based articles popular in a number of outdoor
applications such as windows, panels, and signage. Recently,
acrylic films also have been suggested for use in optical displays.
In this regard, acrylic films are intended to replace glass to
produce lightweight, flexible optical display screens. These
display screens may be utilized in liquid crystal displays, OLED
(organic light emitting diode) displays, and in other electronic
displays found in, for example, personal computers, televisions,
cell phones, and instrument panels.
Polymers of the acrylic type are available in a variety of
molecular weights as well as a variety of molecular structures. The
basic acrylic monomeric structure is
CH.sub.2.dbd.C(R.sub.1)--COOR.sub.2. Depending on the nature of the
R.sub.1 and R.sub.2 groups, a versatile array of acrylic polymers
can be made. In terms of commercially significant acrylics, R.sub.1
is either hydrogen (acrylates) or a methyl group (methylacrylates)
while the R.sub.2 ester group may range among hydrogen, methyl,
ethyl, or butyl groups among others. From an optics viewpoint,
polymethylmethacrylate (PMMA) having methyl groups in both the
R.sub.1 and R.sub.2 position is of particular interest. PMMA is
highly transparent, durable and inexpensive.
In general, resin films are prepared either by melt extrusion
methods or by casting methods. Melt extrusion methods involve
heating the resin until molten (approximate viscosity on the order
of 100,000 cp), and then applying the hot molten polymer to a
highly polished metal band or drum with an extrusion die, cooling
the film, and finally peeling the film from the metal support. For
many reasons, however, films prepared by melt extrusion are
generally not suitable for optical applications. Principal among
these is the fact that melt extruded films generally exhibit a high
degree of optical birefringence. In the case of many acrylic
polymers, melt extruded films are also known to suffer from a
number of imperfections such as pinholes, dimensional instability,
and gels as described in U.S. Pat. No. 4,584,231 to Knoop. Such
imperfections may compromise the optical and mechanical properties
of acrylic films. For example, undesirably higher haze values have
been noted in acrylic films prepared by the melt extrusion method
as noted in the Handbook of Plastics, Elastomers and Composites,
pp. 6.66 8, C A Harper editor, McGraw-Hill Inc. (2000). For these
reasons, melt extrusion methods are generally not practical for
fabricating many resin films including acrylic films intended for
more demanding optical applications. Rather, casting methods are
generally used to produce optical films.
Resin films for optical applications are manufactured almost
exclusively by casting methods. Casting methods involve first
dissolving the polymer in an appropriate solvent to form a dope
having a high viscosity on the order of 50,000 cp, and then
applying the viscous dope to a continuous highly polished metal
band or drum through an extrusion die, partially drying the wet
film, peeling the partially dried film from the metal support, and
conveying the partially dried film through an oven to more
completely remove solvent from the film. Cast films typically have
a final dry thickness in the range of 40 200 microns. In general,
thin films of less than 40 microns are very difficult to produce by
casting methods due to the fragility of wet film during the peeling
and drying processes. Films having a thickness of greater than 200
microns are also problematic to manufacture due to difficulties
associated with the removal of solvent in the final drying step.
Although the dissolution and drying steps of the casting method add
complexity and expense, cast films generally have better optical
properties when compared to films prepared by melt extrusion
methods, and problems associated with high temperature processing
are avoided.
Examples of optical films prepared by casting methods include: 1.)
Polyvinyl alcohol sheets used to prepare light polarizers as
disclosed in U.S. Pat. No. 4,895,769 to Land and U.S. Pat. No.
5,925,289 to Cael as well as more recent disclosures in U.S. Patent
Application. Serial No. 2001/0039319 A1 to Harita and U.S. Patent
Application Serial No. 2002/001700 A1 to Sanefuji, 2.) Cellulose
triacetate sheets used for protective covers for light polarizers
as disclosed in U.S. Pat. No. 5,695,694 to Iwata, 3.) Polycarbonate
sheets used for protective covers for light polarizers or for
retardation plates as disclosed in U.S. Pat. No. 5,818,559 to
Yoshida and U.S. Pat. Nos. 5,478,518 and 5,561,180 both to
Taketani, and 4.) Polysulfone sheets used for protective covers for
light polarizers or for retardation plates as disclosed in U.S.
Pat. No. 5,611,985 to Kobayashi and U.S. Pat. Nos. 5,759,449 and
5,958,305 both to Shiro.
In general, acrylic films can not be manufactured using the casting
method. This is due to the fact that acrylic films are not easily
stripped or peeled from the casting substrate without tearing the
film. U.S. Pat. Nos. 4,584,231 and 4,664,859 both to Knoop teach
the use of specialty acrylic copolymers to overcome the stripping
problems associated with the manufacture of acrylic films using the
casting method. However, these specialty copolymers and copolymer
blends are complex, expensive, and not suitable for preparing high
quality films for demanding optical applications. For example, the
copolymer systems suggested in U.S. Pat. Nos. 4,584,231 and
4,664,859 both to Knoop rely on the use of soft polymer segments.
These soft segments are known to reduce the continuous service
temperature and abrasion resistance of acrylic materials.
Despite the wide use of the casting method to manufacture optical
films, however, there are a number of disadvantages to casting
technology. One disadvantage is that cast films have significant
optical birefringence. Although films prepared by casting methods
have lower birefringence when compared to films prepared by melt
extrusion methods, birefringence remains objectionably high. For
example, cellulose triacetate films prepared by casting methods
exhibit in-plane retardation of 7 nanometers (nm) for light in the
visible spectrum as disclosed in U.S. Pat. No. 5,695,694 to Iwata.
Polycarbonate films prepared by casting methods exhibit in-plane
retardation of 17 nm as disclosed in U.S. Pat. Nos. 5,478,518 and
5,561,180 both to Taketani. U.S. Patent Application. Serial no.
2001/0039319 A1 to Harita claims that color irregularities in
stretched polyvinyl alcohol sheets are reduced when the difference
in retardation between widthwise positions within the film is less
than 5 nm in the original unstretched film. For many applications
of optical films, low in-plane retardation values are desirable. In
particular, values of in-plane retardation of less than 10 nm are
preferred.
Birefringence in cast films arises from orientation of polymers
during the manufacturing operations. This molecular orientation
causes indices of refraction within the plane of the film to be
measurably different. In-plane birefringence is the difference
between these indices of refraction in perpendicular directions
within the plane of the film. The absolute value of birefringence
multiplied by the film thickness is defined as in-plane
retardation. Therefore, in-plane retardation is a measure of
molecular anisotropy within the plane of the film.
During the casting process, molecular orientation may arise from a
number of sources including shear of the dope in the die, shear of
the dope by the metal support during application, shear of the
partially dried film during the peeling step, and shear of the
free-standing film during conveyance through the final drying step.
These shear forces orient the polymer molecules and ultimately give
rise to undesirably high birefringence or retardation values. To
minimize shear and obtain the lowest birefringence films, casting
processes are typically operated at very low line speeds of 1 15
m/min as disclosed in U.S. Pat. No. 5,695,694 to Iwata. Slower line
speeds generally produce the highest quality films.
Another drawback to the casting method is the inability to
accurately apply multiple layers. As noted in U.S. Pat. No.
5,256,357 to Hayward, conventional multi-slot casting dies create
unacceptably non-uniform films. In particular, line and streak
non-uniformity is greater than 5% with prior art devices.
Acceptable two layer films may be prepared by employing special die
lip designs as taught in U.S. Pat. No. 5,256,357 to Hayward, but
the die designs are complex and may be impractical for applying
more than two layers simultaneously.
Another drawback to the casting method is the restrictions on the
viscosity of the dope. In casting practice, the viscosity of dope
is on the order of 50,000 cp. For example, U.S. Pat. No. 5,256,357
to Hayward describes practical casting examples using dopes with a
viscosity of 100,000 cp. In general, cast films prepared with lower
viscosity dopes are known to produce non-uniform films as noted for
example in U.S. Pat. No. 5,695,694 to Iwata. In U.S. Pat. No.
5,695,694 to Iwata, the lowest viscosity dopes used to prepare
casting samples are approximately 10,000 cp. At these high
viscosity values, however, casting dopes are difficult to filter
and degas. While fibers and larger debris may be removed, softer
materials such as polymer slugs are more difficult to filter at the
high pressures found in dope delivery systems. Particulate and
bubble artifacts create conspicuous inclusion defects as well as
streaks and may create substantial waste.
In addition, the casting method can be relatively inflexible with
respect to product changes. Because casting requires high viscosity
dopes, changing product formulations requires extensive down time
for cleaning delivery systems to eliminate the possibility of
contamination. Particularly problematic are formulation changes
involving incompatible polymers and solvents. In fact, formulation
changes are so time consuming and expensive with the casting method
that most production machines are dedicated exclusively to
producing only one film type.
Finally, cast films may exhibit undesirable cockle or wrinkles.
Thinner films are especially vulnerable to dimensional artifacts
either during the peeling and drying steps of the casting process
or during subsequent handling of the film. In particular, the
preparation of composite optical plates from resin films requires a
lamination process involving application of adhesives, pressure,
and high temperatures. Very thin films are difficult to handle
during this lamination process without wrinkling. In addition, many
cast films may naturally become distorted over time due to the
effects of moisture. For optical films, good dimensional stability
is necessary during storage as well as during subsequent
fabrication of composite optical plates.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to overcome the
limitations of prior art casting methods and provide a new coating
method for preparing amorphous acrylic films having very low
in-plane birefringence.
It is a further object of the present invention to provide a new
method of producing highly uniform acrylic films over a broad range
of dry thicknesses.
Yet another object of the present invention is to provide a method
of preparing acrylic films by simultaneously applying multiple
layers to a moving substrate.
Still another object of the present invention is to provide a new
method of preparing acrylic films with improved dimensional
stability and handling ability by temporarily adhering the acrylic
film to a supporting carrier substrate at least until it is
substantially dry and then subsequently separating the carrier
substrate from the acrylic film.
A further object of the present invention is to overcome the
limitations of the prior art casting method and define a new
coating method for preparing freestanding acrylic films without the
need for complex copolymers or copolymer blends.
Briefly stated, the foregoing and numerous other features, objects
and advantages of the present invention will become readily
apparent upon review of the detailed description, claims and
drawings set forth herein. These features, objects and advantages
are accomplished by applying a low viscosity fluid containing
acrylic resin onto a moving carrier substrate by a coating method.
The acrylic film is not separated from the carrier substrate until
the coated film is substantially dry (<10% residual solvent by
weight). In fact, the composite structure of acrylic film and
carrier substrate may be wound into rolls and stored until needed.
Thus, the carrier substrate cradles the acrylic film and protects
against shearing forces during conveyance through the drying
process. Moreover, because the acrylic film is dry and solid when
it is finally peeled from the carrier substrate, there is no shear
or orientation of polymer within the film due to the peeling
process. As a result, acrylic resin films prepared by the current
invention are remarkably amorphous and exhibit very low in-plane
birefringence.
Acrylic films can be made with the method of the present invention
having a thickness of about 1 to 500 .mu.m. Very thin acrylic films
of less than 40 microns can be easily manufactured at line speeds
not possible with prior art methods. The fabrication of very thin
films is facilitated by a carrier substrate that supports the wet
film through the drying process and eliminates the need to peel the
film from a metal band or drum prior to a final drying step as
required in the casting methods described in prior art. Rather, the
acrylic film is substantially, if not completely, dried before
separation from the carrier substrate. In all cases, dried acrylic
films have a residual solvent content of less than 10% by weight.
In a preferred embodiment of the present invention, the residual
solvent content is less than 5%, and most preferably less than 1%.
Thus, the present invention readily allows for preparation of very
delicate thin films not possible with the prior art casting method.
In addition, thick films of greater than 40 microns may also be
prepared by the method of the present invention. To fabricate
thicker films, additional coatings may be applied over a
film-substrate composite either in a tandem operation or in an
offline process without comprising optical quality. In this way,
the method of the present invention overcomes the limitation of
solvent removal during the preparation of thicker films since the
first applied film is dry before application of a subsequent wet
film. Thus, the present invention allows for a broader range of
final film thickness than is possible with casting methods.
In the method of the present invention, acrylic films are created
by forming a single or, preferably, a multi-layer composite on a
slide surface of a coating hopper, the multi-layer composite
including a bottom layer of low viscosity, one or more intermediate
layers, and an optional top layer containing a surfactant, flowing
the multi-layer composite down the slide surface and over a coating
lip of the coating hopper, and applying the multi-layer composite
to a moving substrate. In particular, the use of the method of the
present invention is shown to allow for application of several
liquid layers having unique composition. Coating aids and additives
may be placed in specific layers to improve film performance or
improve manufacturing robustness. For example, multi-layer
application allows a surfactant to be placed in the top spreading
layer, where needed, rather than through out the entire wet film.
In another example, the concentration of acrylic polymer in the
lowermost layer may be adjusted to achieve low viscosity and
facilitate high-speed application of the multi-layer composite onto
the carrier substrate. Therefore, the present invention provides an
advantageous method for the fabrication of multiple layer composite
films such as required for certain optical elements or other
similar elements.
Wrinkling and cockle artifacts are minimized with the method of the
present invention through the use of the carrier substrate. By
providing a stiff backing for the acrylic film, the carrier
substrate minimizes dimensional distortion of the acrylic resin
film. This is particularly advantageous for handling and processing
very thin films of less than about 40 microns. In addition, the
restraining nature of the carrier substrate also eliminates the
tendency of acrylic films to distort or cockle over time as a
result of changes in moisture levels. Thus, the method of the
current invention insures that acrylic films are dimensionally
stable during preparation and storage as well as during final
handling steps necessary for fabrication of optical elements.
The method of the present invention also does not require the use
of specialty copolymers or blends of copolymers containing soft
segments to allow peeling of the film from the substrate as
required in casting operations. Because the film is peeled from the
substrate only after the film is substantially dry, a broader range
of acrylic polymeric film types is possible. As a result, acrylic
films of pure, high molecular weight polymethylmethacrylate (PMMA)
are readily manufactured with the method of the present invention.
Among the acrylics, high molecular weight PMMA produces films
having exceptional clarity, abrasion resistance, and
durability.
In the practice of the method of the present invention, it is
preferred that the substrate be a discontinuous sheet such as
polyethylene terephthalate (PET). The PET carrier substrate may be
pretreated with a subbing layer or an electrical discharge device
to modify adhesion between the acrylic film and the PET substrate.
In particular, a subbing layer or electrical discharge treatment
may enhance the adhesion between the film and the substrate, but
still allow the film to be subsequently peeled away from the
substrate.
Although the present invention is discussed herein with particular
reference to a slide bead coating operation, those skilled in the
art will understand that the present invention can be
advantageously practiced with other coating operations. For
example, freestanding films having low in-plane retardation may be
achievable with single or multiple layer slot die coating
operations and single or multiple layer curtain coating operations.
Moreover, those skilled in the art will recognize that the present
invention can be advantageously practiced with alternative carrier
substrates. For example, peeling films having low in-plane
birefringence may be achievable with other resin supports [e.g.
polyethylene naphthalate (PEN), cellulose acetate, polycarbonate,
PET], paper supports, resin treated paper supports, and metal
supports (e.g. aluminum). Practical applications of the present
invention include the preparation of acrylic sheets used for
optical films (e.g. protective covers, and substrates), laminate
films, release films, photographic films, and packaging films among
others. In particular, acrylic sheets prepared by the method of the
present invention may be utilized as optical films in the
manufacture of electronic displays such as liquid crystal displays.
For example, liquid crystal displays are comprised of a number of
film elements including polarizer plates, compensation plates and
electrode substrates. Polarizer plates are typically a multi-layer
composite structure having dichroic film (normally stretched
polyvinyl alcohol treated with iodine) with each surface adhered to
a protective cover. The acrylic films prepared by the method of the
present invention are suitable as protective covers for polarizer
plates. The acrylic films prepared by the method of the present
invention are also suitable for the manufacture of compensation
plates.
The acrylic film produced with the method of the present invention
is an optical film. As produced, the acrylic film made with the
method of the present invention will have a light transmittance of
at least about 85 percent, preferably at least about 90 percent,
and most preferably, at least about 95 percent. Further, as
produced the acrylic film will have a haze value of less than 1.0
percent. In addition, the acrylic films are smooth with a surface
roughness average of less than 100 nm and most preferrably with a
surface roughness of less than 50 nm.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic of an exemplary coating and drying apparatus
that can be used in the practice of the method of the present
invention.
FIG. 2 is a schematic of an exemplary coating and drying apparatus
of FIG. 1 including a station where the acrylic web separated from
the substrate is separately wound.
FIG. 3 is a schematic of an exemplary multi-slot coating apparatus
that can be used in the practice of the method of the present
invention.
FIG. 4 shows a cross-sectional representation of a single-layer
acrylic film partially peeled from a carrier substrate and formed
by the method of the present invention.
FIG. 5 shows a cross-sectional representation of a single-layer
acrylic film partially peeled from a carrier substrate and formed
by the method of the present invention wherein the carrier
substrate has a subbing layer formed thereon.
FIG. 6 shows a cross-sectional representation of a multi-layer
acrylic film partially peeled from a carrier substrate and formed
by the method of the present invention.
FIG. 7 shows a cross-sectional representation of a multi-layer
acrylic film partially peeled from a carrier substrate and formed
by the method of the present invention wherein the carrier
substrate has a subbing layer formed thereon.
FIG. 8 is a schematic of a casting apparatus as used in prior art
to cast films.
DETAILED DESCRIPTION OF THE INVENTION
Turning first to FIG. 1 there is shown a schematic of an exemplary
and well known coating and drying system 10 suitable for practicing
the method of the present invention. The coating and drying system
10 is typically used to apply very thin films to a moving substrate
12 and to subsequently remove solvent in a dryer 14. A single
coating apparatus 16 is shown such that system 10 has only one
coating application point and only one dryer 14, but two or three
(even as many as six) additional coating application points with
corresponding drying sections are known in the fabrication of
composite thin films. The process of sequential application and
drying is known in the art as a tandem coating operation.
Coating and drying apparatus 10 includes an unwinding station 18 to
feed the moving substrate 12 around a back-up roller 20 where the
coating is applied by coating apparatus 16. The coated web 22 then
proceeds through the dryer 14. In the practice of the method of the
present invention, the final dry film 24, comprising an acrylic
resin film on substrate 12, is wound into rolls at a wind-up
station 26.
As depicted, an exemplary four-layer coating is applied to moving
web 12. Coating liquid for each layer is held in respective coating
supply vessels 28, 30, 32, 34. The coating liquid is delivered by
pumps 36, 38, 40, 42 from the coating supply vessels to the coating
apparatus 16 conduits 44, 46, 48, 50, respectively. In addition,
coating and drying system 10 may also include electrical discharge
devices, such as corona or glow discharge device 52, or polar
charge assist device 54, to modify the substrate 12 prior to
application of the coating.
Turning next to FIG. 2 there is shown a schematic of the same
exemplary coating and drying system 10 depicted in FIG. 1 with an
alternative winding operation. Accordingly, the drawings are
numbered identically up to the winding operation. In the practice
of the method of the present invention the dry film 24 comprising a
substrate (which may be a resin film, paper, resin coated paper or
metal) with an acrylic coating applied thereto is taken between
opposing rollers 56, 58. The acrylic film 60 is peeled from
substrate 12 with the acrylic film going to winding station 62 and
the substrate 12 going to winding station 64. In a preferred
embodiment of the present invention, polyethylene terephthalate
(PET) is used as the substrate 12. The substrate 12 may be
pretreated with a subbing layer to enhance adhesion of the coated
film 60 to the substrate 12.
The coating apparatus 16 used to deliver coating fluids to the
moving substrate 12 may be a multi-layer applicator such as a slide
bead hopper, as taught for example in U.S. Pat. No. 2,761,791 to
Russell, or a slide curtain hopper, as taught by U.S. Pat. No.
3,508,947 to Hughes. Alternatively, the coating apparatus 16 may be
a single layer applicator, such as a slot die hopper or a jet
hopper. In a preferred embodiment of the present invention, the
application device 16 is a multi-layer slide bead hopper.
As mentioned above, coating and drying system 10 includes a dryer
14 that will typically be a drying oven to remove solvent from the
coated film. An exemplary dryer 14 used in the practice of the
method of the present invention includes a first drying section 66
followed by eight additional drying sections 68 82 capable of
independent control of temperature and air flow. Although dryer 14
is shown as having nine independent drying sections, drying ovens
with fewer compartments are well known and may be used to practice
the method of the present invention. In a preferred embodiment of
the present invention the dryer 14 has at least two independent
drying zones or sections. Preferably, each of drying sections 68 82
each has independent temperature and airflow controls. In each
section, temperature may be adjusted between 5.degree. C. and
150.degree. C. In the practice of the method of the present
invention, initial drying sections 66, 68 should be operated at
temperatures of at least about 25.degree. C. but less than
95.degree. C. It is preferred that initial drying sections 66, 68
be operated at temperatures between about 30.degree. C. and about
60.degree. C. It is most preferred that initial drying sections 66,
68 be operated at temperatures between about 30.degree. C. and
about 50.degree. C. The actual drying temperature in drying
sections 66, 68 may optimized empirically within these ranges by
those skilled in the art.
Referring now to FIG. 3, a schematic of an exemplary coating
apparatus 16 is shown in detail. Coating apparatus 16,
schematically shown in side elevational cross-section, includes a
front section 92, a second section 94, a third section 96, a fourth
section 98, and a back plate 100. There is an inlet 102 into second
section 94 for supplying coating liquid to first metering slot 104
via pump 106 to thereby form a lowermost layer 108. There is an
inlet 110 into third section 96 for supplying coating liquid to
second metering slot 112 via pump 114 to form layer 116. There is
an inlet 118 into fourth section 98 for supplying coating liquid to
metering slot 120 via pump 122 to form layer 124. There is an inlet
126 into back plate 100 for supplying coating liquid to metering
slot 128 via pump 130 to form layer 132. Each slot 104, 112, 120,
128 includes a transverse distribution cavity. Front section 92
includes an inclined slide surface 134, and a coating lip 136.
There is a second inclined slide surface 138 at the top of second
section 94. There is a third inclined slide surface 140 at the top
of third section 96. There is a fourth inclined slide surface 142
at the top of fourth section 98. Back plate 100 extends above
inclined slide surface 142 to form a back land surface 144.
Residing adjacent the coating apparatus or hopper 16 is a coating
backing roller 20 about which a web 12 is conveyed. Coating layers
108, 116, 124, 132 form a multi-layer composite which forms a
coating bead 146 between lip 136 and substrate 12. Typically, the
coating hopper 16 is movable from a non-coating position toward the
coating backing roller 20 and into a coating position. Although
coating apparatus 16 is shown as having four metering slots,
coating dies having a larger number of metering slots (as many as
nine or more) are well known and may be used to practice the method
of the present invention.
In the method of the present invention, the coating fluids are
comprised principally of an acrylic resin dissolved in an organic
solvent. Polymers of the acrylic type are available in a variety of
molecular weights as well as a variety of molecular structures. The
basic monomeric structure of all acrylic polymers is
CH.sub.2.dbd.C(R.sub.1)--COOR.sub.2. Depending on the nature of the
R.sub.1 and R.sub.2 groups, a versatile array of acrylic polymers
are available. In terms of commercially significant acrylics,
R.sub.1 is either hydrogen (acrylates) or a methyl group
(methylacrylates). The R.sub.2 ester group may range among
hydrogen, methyl, ethyl, butyl, lauryl, or stearyl, groups. In
addition, the R.sub.2 ester group may contain reactive hydroxyl
functionality such as when R.sub.2 is a 2-hydroethyl or a
hydroxypropyl group. These reactive components in the R.sub.2
position may allow for crosslinking or hardening of acrylic films
during the coating operation. Suitable hardening agents for these
hydroxyl-substituted groups include isocyantes and borates. From an
optics viewpoint, polymethylmethacrylate (PMMA) having methyl
groups in both the R.sub.1 and R.sub.2 position is of particular
interest. PMMA is highly transparent, durable and inexpensive.
Perhaps more than any other plastic resin, PMMA is used as a
replacement for glass. In the method of the present invention, high
molecular weight PMMA is the preferred acrylic polymer. In terms of
polymer molecular weight, films prepared with lower molecular
weight PMMA are found to be unacceptable brittle. Although films
made with PMMA having a weight average molecular weight of 35,500
daltons had good appearance while supported by the carrier
substrate, the films fractured easily during peeling. For higher
molecular weight PMMA of 455,000 daltons, however, films not only
had a good appearance on the carrier substrate, but peeled well
without fracturing.
In terms of organic solvents for acrylics, suitable sovlents
include, for example, chlorinated solvents (methylene chloride and
1,2 dichloroethane), higher alcohols (n-propanol, isopropanol,
n-butanol, isobutanol, isoamyl alcohol), ketones (acetone,
methylethyl ketone, methylisobutyl ketone, and cyclohexanone,
diacetone alcohol), esters (methyl acetate, ethyl acetate, n-propyl
acetate, isopropyl acetate, isobutyl acetate, and n-butyl acetate),
aromatics (toluene and xylenes) and ethers (1,3-dioxolane and
tetrahydrofuran). Acrylic solutions may be prepared with a blend of
the aforementioned solvents. Preferred primary solvents for high
molecular weight polymethylmethacrylate include methylethhyl
ketone, methylene chloride, ethyl acetate, tetrahydrofuran and
toluene.
Coating fluids may also contain plasticizers. Appropriate
plasticizers for acrylic films include phthalate esters
(isooctylbenzylphthalate, benzylphthalate, butylbenzylphthalate,
diethylphthalate, dibutylphthalate, dioctylphthalate,
didecylphthalate and butyl octylphthalate), and phosphate esters
(tricresyl phosphate). Plasticizers are normally used to improve
the physical and mechanical properties of the final film. In
particular, plasticizers are known to improve the flexibility of
acrylic films. However, plasticizers may also be used here as
coating aids in the converting operation to minimize premature film
solidification at the coating hopper and to improve drying
characteristics of the wet film. In the method of the present
invention, plasticizers may be used to minimize blistering, curl
and delamination of acrylic films during the drying operation.
Plasticizers may be added to the coating fluid at a total
concentration of up to 25% by weight relative to the concentration
of polymer in order to mitigate defects in the final acrylic
film.
Coating fluids may also contain surfactants as coating aids to
control artifacts related to flow after coating. Artifacts created
by flow after coating phenomena include mottle, repellencies,
orange-peel (Bernard cells), and edge-withdraw. Surfactants used
control flow after coating artifacts include siloxane and
fluorochemical compounds. Examples of commercially available
surfactants of the siloxane type include: 1.) Polydimethylsiloxanes
such as DC200 Fluid from Dow Corning, 2.) Poly(dimethyl,
methylphenyl)siloxanes such as DC510 Fluid from Dow Corning, and
3.) Polyalkyl substituted polydimethysiloxanes such as DC190 and
DC1248 from Dow Corning as well as the L7000 Silwet series (L7000,
L7001, L7004 and L7230) from Union Carbide, and 4.) Polyalkyl
substituted poly(dimethyl, methylphenyl)siloxanes such as SF1023
from General Electric. Examples of commercially available
fluorochemical surfactants include: 1.) Fluorinated alkyl esters
such as the Fluorad series (FC430 and FC431) from the 3M
Corporation, 2.) Fluorinated polyoxyethylene ethers such as the
Zonyl series (FSN, FSN100, FSO, FSO100) from Du Pont, 3.)
Acrylate:polyperfluoroalkyl ethylacrylates such as the F series
(F270 and F600) from NOF Corporation, and 4.) Perfluoroalkyl
derivatives such as the Surflon series (S383, S393, and S8405) from
the Asahi Glass Company. In the method of the present invention,
surfactants are generally of the non-ionic type. In a preferred
embodiment of the present invention, non-ionic compounds of the
fluorinated type are added to the uppermost layers.
In terms of surfactant distribution, surfactants are most effective
when present in the uppermost layers of the multi-layer coating. In
the uppermost layer, the concentration of surfactant is preferably
0.001 1.000% by weight and most preferably 0.010 0.500%. In
addition, lesser amounts of surfactant may be used in the second
uppermost layer to minimize diffusion of surfactant away from the
uppermost layers. The concentration of surfactant in the second
uppermost layer is preferably 0.000 0.200% by weight and most
preferably between 0.000 0.100% by weight. Because surfactants are
only necessary in the uppermost layers, the overall amount of
surfactant remaining in the final dried film is small.
Although surfactants are not required to practice the method of the
current invention, surfactants do improve the uniformity of the
coated film. In particular, mottle nonuniformities are reduced by
the use of surfactants. In transparent acrylic films, mottle
nonuniformities are not readily visualized during casual
inspection. To visualize mottle artifacts, organic dyes may be
added to the uppermost layer to add color to the coated film. For
these dyed films, nonuniformities are easy to see and quantify. In
this way, effective surfactant types and levels may be selected for
optimum film uniformity.
Tuning next to FIGS. 4 through 7, there are presented
cross-sectional illustrations showing various film configurations
prepared by the method of the present invention. In FIG. 4, a
single-layer acrylic film 150 is shown partially peeled from a
carrier substrate 152. Acrylic film 150 may be formed either by
applying a single liquid layer to the carrier substrate 152 or by
applying a multiple layer composite having a composition that is
substantially the same among the layers. Alternatively in FIG. 5,
the carrier substrate 154 may have been pretreated with a subbing
layer 156 that modifies the adhesive force between the single layer
acrylic film 158 and the substrate 154. FIG. 6 illustrates a
multiple layer film 160 that is comprised of four compositionally
discrete layers including a lowermost layer 162 nearest to the
carrier support 170, two intermediate layers 164, 166, and an
uppermost layer 168. FIG. 6 also shows that the entire multiple
layer composite 160 may be peeled from the carrier substrate 170.
FIG. 7 shows a multiple layer composite film 172 comprising a
lowermost layer 174 nearest to the carrier substrate 182, two
intermediate layers 176, 178, and an uppermost layer 180 being
peeled from the carrier substrate 182. The carrier substrate 182
has been treated with a subbing layer 184 to modify the adhesion
between the composite film 172 and substrate 182. Subbing layers
156 and 184 may be comprised of a number of polymeric materials
such as polyvinylbutyrals, cellulosics, polyacrylates,
polycarbonates and gelatin. In a preferred embodiment of the
present invention, the carrier substrate is untreated PET. The
choice of materials used in the subbing layer may be optimized
empirically by those skilled in the art.
The method of the present invention may also include the step of
coating over a previously prepared composite of acrylic film and
carrier substrate. For example, the coating and drying system 10
shown in FIGS. 1 and 2 may be used to apply a second multi-layer
film to an existing acrylic film/substrate composite. If the
film/substrate composite is wound into rolls before applying the
subsequent coating, the process is called a multi-pass coating
operation. If coating and drying operations are carried out
sequentially on a machine with multiple coating stations and drying
ovens, then the process is called a tandem coating operation. In
this way, thick films may be prepared at high line speeds without
the problems associated with the removal of large amounts of
solvent from a very thick wet film. Moreover, the practice of
multi-pass or tandem coating also has the advantage of minimizing
other artifacts such as streak severity, mottle severity, and
overall film nonuniformity.
The practice of tandem coating or multi-pass coating requires some
minimal level of adhesion between the first-pass film and the
carrier substrate. In some cases, film/substrate composites having
poor adhesion are observed to blister after application of a second
or third wet coating in a multi-pass operation. To avoid blister
defects, adhesion must be greater than 0.3 N/m between the
first-pass film and the carrier substrate. This level of adhesion
may be attained by a variety of web treatments including various
subbing layers and various electronic discharge treatments.
However, excessive adhesion between the applied film and substrate
is also undesirable since the film may be damaged during subsequent
peeling operations. In particular, film/substrate composites having
an adhesive force of greater than 250 N/m have been found to peel
poorly. Films peeled from such excessively well-adhered composites
exhibit defects due to tearing of the film and/or due to cohesive
failure within the film. In a preferred embodiment of the present
invention, the adhesion between the acrylic film and the carrier
substrate is less than 250 N/m. Most preferably, the adhesion
between polycarbonate film and the carrier substrate is between 0.5
and 25 N/m.
The method of the present invention is suitable for application of
acrylic resin coatings to a variety of substrates such as
polyethylene terephthalate (PET), polyethylene naphthalate (PEN),
polycarbonate, polystyrene, and other polymeric films. Additional
substrates may include paper, laminates of paper and polymeric
films, glass, cloth, aluminum and other metal supports. In some
cases, substrates may be pretreated with subbing layers or
electrical discharge devices. Substrates may also be pretreated
with functional layers containing various binders and addenda.
The prior art method of casting resin films is illustrated in FIG.
8. As shown in FIG. 8, a viscous polymeric dope is delivered
through a feed line 200 to an extrusion hopper 202 from a
pressurized tank 204 by a pump 206. The dope is cast onto a highly
polished metal drum 208 located within a first drying section 210
of the drying oven 212. The cast film 214 is allowed to partially
dry on the moving drum 208 and is then peeled from the drum 208.
The cast film 214 is then conveyed to a final drying section 216 to
remove the remaining solvent. The final dried film 218 is then
wound into rolls at a wind-up station 220. The prior art cast film
typically has a thickness in the range of from 40 to 200 .mu.m.
Coating methods are distinguished from casting methods by the
process steps necessary for each technology. These process steps in
turn affect a number of tangibles, such as fluid viscosity,
converting aids, substrates, and hardware that are unique to each
method. In general, coating methods involve application of dilute
low viscosity liquids to thin flexible substrates, evaporating the
solvent in a drying oven, and winding the dried film/substrate
composite into rolls. In contrast, casting methods involve applying
a concentrated viscous dope to a highly polished metal drum or
band, partially drying the wet film on the metal substrate,
stripping the partially dried film from the substrate, removing
additional solvent from the partially dried film in a drying oven,
and winding the dried film into rolls. In terms of viscosity,
coating methods require very low viscosity liquids of less than
5,000 cp. In the practice of the method of the present invention
the viscosity of the coated liquids will generally be less than
2000 cp and most often less tan 1500 cp. Moreover, in the method of
the present invention the viscosity of the lowermost layer is
preferred to be less than 200 cp. and most preferably less than 100
cp. for high speed coating application. In contrast, casting
methods require highly concentrated dopes with viscosity on the
order of 10,000 100,000 cp for practical operating speeds. In terms
of converting aids, coating methods generally involve the use of
surfactants as converting aids to control flow after coating
artifacts such as mottle, repellencies, orange peel, and edge
withdraw. In contrast, casting methods do not require surfactants.
Instead, converting aids are only used to assist in the stripping
operation in casting methods. For example, n-butanol and water are
sometimes used as a converting aid in casting films of cellulose
acetate or polysulfones to facilitate stripping of the film from
the metal drum or band. In terms of substrates, coating methods
generally utilize thin (10 250 micron) flexible supports. In
contrast, casting methods employ thick (1 100 mm), continuous,
highly polished metal drums or rigid bands. As a result of these
differences in process steps, the hardware used in coating is
conspicuously different from those used in casting as can be seen
by a comparison of the schematics shown in FIGS. 1 and 8,
respectively.
The advantages of the present invention are demonstrated by the
following practical examples given below. In these examples, the
acrylic polymer was the polymethylmethacrylate type (PMMA) with a
weight average molecular weight of 455,000 daltons unless otherwise
noted.
EXAMPLE 1
This example describes the single pass formation of a very thin
acrylic film. The coating apparatus 16 illustrated in FIG. 1 was
used to apply four liquid layers to a moving substrate 12, 170 of
untreated polyethylene terephthalate (PET) to form a single layer
film as illustrated earlier in FIG. 6. The substrate speed was 25
cm/s. All coating fluids were comprised of PMMA dissolved in
solvent system of 8:2 methylethylketone:toluene where the ratio is
by weight. The lowermost layer 162 had a viscosity of 23 cp. and a
wet thickness of 11 .mu.m on the moving substrate 170. The second
164 and third 166 layers each had a viscosity of 820 cp. and had a
combined final wet thickness of 66 .mu.m on the moving substrate
170. In addition, the third layer 166 also contained a fluorinated
surfactant (Surflon S8405) at concentration of 0.05%. The uppermost
layer 168 had a viscosity of 195 cp. and a wet thickness of 22
.mu.m on the moving substrate 170. The uppermost layer 168 also
contained a fluorinated surfactant (Surflon S8405) at a weight
percent of 0.20%. Coatings were applied at a temperature of
24.degree. C. The gap between the coating lip 136 and the moving
substrate 12 (see FIG. 3) was 200 .mu.m. The pressure differential
across the coating bead 146 was adjusted between 0 10 cm of water
to establish a uniform coating. The temperature in the drying
sections 66 and 68 was 25.degree. C. The temperature in the drying
section 70 was 50.degree. C. The temperature in the drying sections
72, 74, 76, 78, 80 was 120.degree. C. The temperature in the drying
section 82 was 25.degree. C. The composite of PMMA film and PET
substrate was wound into rolls. The composite of PMMA film and PET
substrate was free from wrinkles and cockle artifacts. When peeled
from the untreated PET substrate, the final dry film had a
thickness of 10 .mu.m. The peeled PMMA film had a good appearance,
was smooth, and had an in-plane retardation of less than 1.0 nm.
Properties of this acrylic film are summarized in Table I.
EXAMPLE 2
This example describes the single pass formation of a thin PMMA
film. The conditions were identical to those described in Example 1
except that the combined wet thickness of the second and third
layers 164 and 166 was increased to 147 am. The composite of PMMA
film and PET substrate was wound into rolls. The composite of PMMA
film and PET substrate was free from wrinkles and cockle artifacts.
When peeled from the subbed PET substrate, the final dry film had a
thickness of 20 .mu.m. The peeled PMMA film had a good appearance,
was smooth, and had an in-plane retardation of less than 1.0 nm.
Properties of this PMMA film are summarized in Table I.
EXAMPLE 3
This example describes the single pass formation of a thin PMMA
film. The conditions were identical to those described in Example 1
except that the combined wet thickness of the second and third
layers 164 and 166 was increased to 228 .mu.m. The composite of
PMMA film and PET substrate was wound into rolls. The composite of
PMMA film and PET substrate was free from wrinkles and cockle
artifacts. When peeled from the subbed PET substrate, the final dry
film had a thickness of 30 .mu.m. The PMMA film had a good
appearance, was smooth, and had an in-plane retardation of less
than 1.0 nm. Properties of this PMMA film are summarized in Table
I.
EXAMPLE 4
This example describes the single pass formation a PMMA film. The
conditions were identical to those described in Example 3 except
that the combined wet thickness of the second and third layers 164
and 166 was increased to 309 .mu.m. The composite of PMMA film and
PET substrate was wound into rolls. The composite of PMMA film and
PET substrate was free from wrinkles and cockle artifacts. When
peeled from the subbed PET substrate, the final dry film had a
thickness of 40 .mu.m. The peeled PMMA film had a good appearance,
was smooth, and had an in-plane retardation of less than 1.0 nm.
Properties of this PMMA film are summarized in Table I.
EXAMPLE 5
This example describes the formation of a PMMA film using a
two-pass coating operation. The conditions were identical to those
described in Example 4 except that the wound composite of PMMA film
and PET substrate of Example 4 was subsequently over-coated with an
additional pass. The second pass was conducted with the combined
wet thickness of the second and third layers at 309 .mu.m as
described in Example 4. The final composite of PMMA film and PET
substrate was wound into rolls. The composite of PMMA film and PET
substrate was free from wrinkles and cockle artifacts. The final
dry film had a thickness of 80 .mu.m. The peeled PMMA film was
smooth, and had an in-plane retardation of less than 1.0 nm.
Properties of this PMMA film are summarized in Table I.
COMPARATIVE EXAMPLE 1
This example describes the formation of a PMMA: PET composite
having poor peeling properties. In this example, the PET support
has a subbing layer of poly(acrylonitrile-co-vinylidene
chloride-co-acrylic acid) at a dry coverage of 100 mg/sq-m.
Otherwise, the conditions for Comparative Example 1 were identical
to those described in Example 2. The final dry film had a thickness
of 20 .mu.m. When dried, the PMMA film could not be peeled from the
subbed PET substrate without tearing the film. When measured
analytically the adhesive force of the PMMA film to the subbed PET
substrate was found to be greater than 250 N/m.
COMPARATIVE EXAMPLE 2
This example describes defects formed as a result of poor drying
conditions during a single pass operation. The conditions for
Comparative Example 2 were identical to those described in Example
2 except that the drying conditions were adjusted such that the
temperature in the first three drying zones 66, 68, 70 was
increased to 95.degree. C. When peeled from the subbed PET
substrate, the final dry film had a thickness of 20 .mu.m. The
peeled PMMA film was of unacceptable quality due to a reticulation
pattern in the film.
COMPARATIVE EXAMPLE 3
This example describes defects formed as a result of replacement of
the high molecular weight acrylic used in the above examples with a
lower molecular weight PMMA of 35,500 daltons. For these
experiments, the conditions of Example 1 through Example 4 were
repeated using the lower molecular weight PMMA in a single pass
operation. The PMMA:PET composite films were wound into rolls. The
final PMMA film thickness was 10, 20, 30 and 40 microns. Regardless
of thickness, the lower molecular weight PMMA films could not be
peeled from the carrier substrate without breaking or fracturing
the film. All of these films prepared with the lower molecular
weight PMMA were found to be unacceptably brittle.
TABLE-US-00001 TABLE I Trans- Example Thickness Retardation
mittance Haze Roughness 1 10 .mu.m 0.1 nm 94.3% 0.3% 0.3 nm 2 20
0.1 94.4 0.3 0.4 3 30 0.1 94.3 0.3 0.4 4 40 0.1 94.3 0.3 0.4 5 80
0.3 94.3 0.3 0.8
The following tests were used to determine the film properties
given in Table I.
Thickness. Thickness of the final peeled film was measured in
microns using a Model EG-225 gauge from the Ono Sokki Company.
Retardation. In-plane retardation (R.sub.e) of peeled films were
determined in nanometers (nm) using a Woollam M-2000V Spectroscopic
Ellipsometer at wavelengths from 370 to 1000 nm. In-plane
retardation values in Table I are computed for measurements taken
at 590 nm. In-plane retardation is defined by the formula:
R.sub.e=|n.sub.x-n.sub.y|.times.d where R.sub.e is the in-plane
retardation at 590 nm, n.sub.x is the index of refraction of the
peeled film in the slow axis direction, n.sub.y is the is the index
of refraction of the peeled film in the fast axis direction, and d
is the thickness of the peeled film in nanometers (nm). Thus,
R.sub.e is the absolute value of the difference in birefringence
between the slow axis direction and the fast axis direction in the
plane of the peeled film multiplied by the thickness of the
film.
Transmittance and Haze. Total transmittance and haze are measured
using the Haze-Gard Plus (Model HB-4725) from BYK-Gardner. Total
transmittance is all the light energy transmitted through the film
as absorbed on an integrating sphere. Transmitted haze is all light
energy scattered beyond 2.5.degree. as absorbed on an integrating
sphere.
Surface Roughness. Surface roughness was determined in nanometers
(nm) by scanning probe microscopy using TappingMode.TM. Atomic
Force Microscopy (Model D300 from Digital Instruments).
Adhesion. The adhesion strength of the coated samples was measured
in Newtons per meter (N/m) using a modified 180.degree. peel test
with an Instron 1122 Tensile Tester with a 500 gram load cell.
First, 0.0254 m (one inch) wide strips of the coated sample were
prepared. Delamination of the coating at one end was initiated
using a piece of 3M Magic Tape. An additional piece of tape was
then attached to the delaminated part of the coating and served as
the gripping point for testing. The extending tape was long enough
to extend beyond the support such that the Instron grips did not
interfere with the testing. The sample was then mounted into the
Instron 1122 Tensile Tester with the substrate clamped tin the
upper grip and the coating/tape assembly clamped in the bottom
grip. The average force (in units of Newtons) required to peel the
coating off the substrate at a 180.degree. angle at speed of 2
inches/min (50.8 mm/min) was recorded. Using this force value the
adhesive strength in units of N/m was calculated using the
equation: S.sub.A=F.sub.p(1-cos .theta.)/w wherein S.sub.A is the
adhesive strength, F.sub.p is the peel force, .theta. is the angle
of peel (180.degree.), and w is the width of the sample (0.0254
m).
Residual Solvent. A qualitative assessment of residual solvents
remaining in a dried film is done by first peeling the film from
the carrier substrate, weighing the peeled film, incubating the
film in an oven at 100.degree. C. for 16 hours, and finally
weighing the incubated film. Residual solvent is expressed as
percentage of the weight difference divided by the post-incubation
weight.
From the foregoing, it will be seen that this invention is one well
adapted to obtain all of the ends and objects hereinabove set forth
together with other advantages which are apparent and which are
inherent to the apparatus.
It will be understood that certain features and sub-combinations
are of utility and may be employed with reference to other features
and sub-combinations. This is contemplated by and is within the
scope of the claims.
As many possible embodiments may be made of the invention without
departing from the scope thereof, it is to be understood that all
matter herein set forth and shown in the accompanying drawings is
to be interpreted as illustrative and not in a limiting sense.
TABLE-US-00002 PARTS LIST 10 drying system 12 moving substrate/web
14 dryer 16 coating apparatus 18 unwinding station 20 back-up
roller 22 coated web 24 dry film 26 wind up station 28 coating
supply vessel 30 coating supply vessel 32 coating supply vessel 34
coating supply vessel 36 pumps 38 pumps 40 pumps 42 pumps 44
conduits 46 conduits 48 conduits 50 conduits 52 discharge device 54
polar charge assist device 56 opposing rollers 58 opposing rollers
60 acrylic film 62 winding station 64 winding station 66 drying
section 68 drying section 70 drying section 72 drying section 74
drying section 76 drying section 78 drying section 80 drying
section 82 drying section 92 front section 94 second section 96
third section 98 fourth section 100 back plate 102 inlet 104
metering slot 106 pump 108 lower most layer 110 inlet 112 2.sup.nd
metering slot 114 pump 116 layer 118 inlet 120 metering slot 122
pump 124 form layer 126 inlet 128 metering slot 130 pump 132 layer
134 incline slide surface 136 coating lip 138 2.sup.nd incline
slide surface 140 3.sup.rd incline slide surface 142 4.sup.th
incline slide surface 144 back land surface 146 coating bead 150
acrylic film 152 carrier substrate 154 carrier substrate 156
subbing layer 158 acrylic film 160 multiple layer film 162 lower
most layer 164 intermediate layers 166 intermediate layers 168
upper most layer 170 carrier support 172 composite film 174 lower
most layer 176 intermediate layers 178 intermediate layers 180
upper most layers 182 carrier substrate 184 subbing layer 200 feed
line 202 extrusion hopper 204 pressurized tank 206 pump 208 metal
drum 210 drying section 212 drying oven 214 cast film 216 final
drying section 218 final dried film 220 wind-up station
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